Connecting to the S120

This guide shows you how to establish a BLE connection to the Huepar S120 laser distance meter.

Prerequisites

• Bluetooth adapter with BLE support
• S120 device powered on
• Platform-specific BLE library installed

Device Discovery

The S120 advertises as LDM-S120 xxxx where xxxx is the last 4 hex digits of its MAC address.

Linux (BlueZ)

# Start scanning
bluetoothctl scan on

# Look for output like:
# [NEW] Device 5B:A6:86:38:FA:CA LDM-S120 faca

# Stop scanning
bluetoothctl scan off

Python (bleak)

import asyncio
from bleak import BleakScanner

async def scan():
    devices = await BleakScanner.discover()
    for d in devices:
        if d.name and d.name.startswith('LDM-S120'):
            print(f"Found: {d.name} at {d.address}")

asyncio.run(scan())

Node.js (noble)

const noble = require('@abandonware/noble');

noble.on('discover', (peripheral) => {
  if (peripheral.advertisement.localName?.startsWith('LDM-S120')) {
    console.log(`Found: ${peripheral.advertisement.localName}`);
    console.log(`Address: ${peripheral.address}`);
    noble.stopScanning();
  }
});

noble.startScanning([], true);

Connecting

Linux Requirement

On Linux with BlueZ, you must trust the device before connecting. Otherwise, you’ll get “Authentication Canceled” errors.

Linux (BlueZ)

1. Trust the device

   bluetoothctl trust 5B:A6:86:38:FA:CA

2. Connect

   bluetoothctl connect 5B:A6:86:38:FA:CA

3. Verify connection

   bluetoothctl info 5B:A6:86:38:FA:CA
   # Should show "Connected: yes"

Python (bleak)

import asyncio
from bleak import BleakClient

ADDRESS = "5B:A6:86:38:FA:CA"

async def connect():
    async with BleakClient(ADDRESS) as client:
        print(f"Connected: {client.is_connected}")
        # Your code here

asyncio.run(connect())

Tip

Bleak handles trust/pairing automatically on most platforms. If you get connection errors on Linux, try trusting manually with bluetoothctl trust.

Service Discovery

After connecting, discover GATT services:

async def discover_services(client):
    for service in client.services:
        print(f"Service: {service.uuid}")
        for char in service.characteristics:
            print(f"  Char: {char.uuid} - {char.properties}")

Expected output:

Service: 00001800-0000-1000-8000-00805f9b34fb  (Generic Access)
Service: 00001801-0000-1000-8000-00805f9b34fb  (Generic Attribute)
Service: 0000ae30-0000-1000-8000-00805f9b34fb  (Laser Meter)
  Char: 0000ae01-... - ['write-without-response']
  Char: 0000ae02-... - ['notify']
  Char: 0000ae05-... - ['indicate']

Enable Notifications

Before sending commands, enable notifications on the data characteristic:

CHAR_RX = "0000ae02-0000-1000-8000-00805f9b34fb"

def notification_handler(sender, data):
    print(f"Received: {data.hex()}")

async def setup_notifications(client):
    await client.start_notify(CHAR_RX, notification_handler)

Send a Test Command

Verify the connection by requesting the device MAC:

CHAR_TX = "0000ae01-0000-1000-8000-00805f9b34fb"

async def get_mac(client):
    cmd = bytes([0xF1, 0x03, 0x02, 0x05])  # GET_DEVICE_MAC
    await client.write_gatt_char(CHAR_TX, cmd)
    # Response arrives via notification handler

Expected response:

Received: f103000c356261363836333866616361...
          └─ "5ba68638faca" in ASCII

Complete Example

import asyncio
from bleak import BleakClient

ADDRESS = "5B:A6:86:38:FA:CA"
SERVICE = "0000ae30-0000-1000-8000-00805f9b34fb"
CHAR_TX = "0000ae01-0000-1000-8000-00805f9b34fb"
CHAR_RX = "0000ae02-0000-1000-8000-00805f9b34fb"

def handle_notification(sender, data):
    print(f"← {data.hex()}")

async def main():
    async with BleakClient(ADDRESS) as client:
        print(f"Connected: {client.is_connected}")

        # Enable notifications
        await client.start_notify(CHAR_RX, handle_notification)

        # Get device MAC
        print("Requesting MAC address...")
        await client.write_gatt_char(CHAR_TX, bytes([0xF1, 0x03, 0x02, 0x05]))
        await asyncio.sleep(1)

        # Get firmware version
        print("Requesting firmware version...")
        await client.write_gatt_char(CHAR_TX, bytes([0xF1, 0x03, 0x03, 0x06]))
        await asyncio.sleep(1)

        # Trigger measurement
        print("Triggering measurement...")
        await client.write_gatt_char(CHAR_TX, bytes([0xF1, 0x04, 0x01, 0x01, 0x06]))
        await asyncio.sleep(2)

asyncio.run(main())

Troubleshooting

”Authentication Canceled” (Linux)

# Trust the device first
bluetoothctl trust 5B:A6:86:38:FA:CA

Device Disconnects Immediately

The device may timeout if idle. Send commands within 10 seconds of connecting.

No Notifications Received

Ensure you’ve enabled notifications on 0xAE02:

await client.start_notify(CHAR_RX, handler)

“Device not found”

• Ensure the S120 is powered on (hold measure button)
• Check the device is in range (under 10m)
• Verify Bluetooth is enabled on your system

Next Steps

• Command Reference - All available commands
• Response Reference - Parse responses
• Capturing Traffic - Debug with btsnoop